Fourteen derivatives of cinnamic and acrylic acids were evaluated for their ability to modulate chemiluminescence, evoked by neutrophils that had been exposed to luminol and phorbol-myristate-acetate. Compounds with one or two hydroxyl groups on the phenyl ring demonstrated significant inhibition of the chemiluminescence, but this inhibition was diminished by methoxylation. Saturation of the double bond in the aliphatic chain of cinnamic acid at C6-enhanced the chemiluminescence to a small degree. All three acrylic acid derivatives demonstrated a marked inhibition of the luminol chemiluminescence, indicating that characteristics of the heterocyclic ring is of utmost importance in this activity.